The present invention relates to an arrangement and a method for stabilizing a vehicle combination comprising a towing vehicle and at least one towed vehicle. The arrangement and method is especially suited for vehicle combinations having more than one towed vehicle.
In order to reduce the number of heavy vehicles on the roads, longer vehicle combinations comprising more than one towed vehicle are proposed for the use on regular roads in some countries. Apart from reducing the required number of towing vehicles for a specific load, the energy consumption and the emission of exhaust gases will also be reduced compared with traditional vehicle combinations. Normally, the length and the weight of the vehicle combination are controlled by law and regulations. In some countries, longer and/or heavier vehicle combinations are already allowed under restricted conditions. Such vehicles may comprise several towed vehicles and may be over 50 meters long and more. They are often used in remote areas and for specific purposes. In Australia, road trains comprising more than 4 trailers are used in some states and on some roads. Longer combination vehicles (LCV's) are also used in e.g. USA, Canada and Argentina. All these LCV's are used under strict regulations.
Long vehicle combinations containing several towed vehicles are in general more instable than vehicles having one or a few towed vehicles at high speed. This means that long vehicle combinations are more prone to roll over, jack-knife, trailer swing out and to start skidding. On the other hand, long vehicle combinations are more transport efficient since their load capacity is higher.
One problem with a longer vehicle combination is the stability of the vehicle combination. Even with vehicle combinations having a single towed vehicle, such as a tractor trailer combination, stability problems may arise when braking or turning. One stability problem that may arise is that the trailer starts swinging from side to side. This may happen when the vehicle travels with a relatively high speed and changes lane or drives in curves. The stability of the vehicle combination will normally correct itself when the vehicle travels straight, but this may still affect the traffic around the vehicle, either by bumping in to other vehicles or by scaring drivers in the vicinity. Another type of stability problem arises when the vehicle combination brakes. One such problem is known as jackknifing, in which the trailer will spin around such that the tractor and trailer will resemble a folded pocket knife.
There are several ways of improving the stability of a vehicle combination in order to avoid accidents. Solutions reducing the turning angle for the trailer have been proposed, unsuccessfully. Anti-lock brakes and electronic brake force distribution controlled by an electronic control unit has reduced some types of accidents. Such solutions are mostly designed for a vehicle combination having a single trailer. For a longer vehicle combination having several towed vehicles, the proposed solutions will not suffice.
US 2010/070149 describes a trailer electronic braking system for a road train having a tractor and a plurality of trailers. The braking system includes a braking ECU on each trailer and a communication interface being provided so that the braking ECU on a first trailer and the braking ECU on a second trailer are able to communicate with one another. In use, the respective braking ECU on the first and second trailer receive an input from a respective sensor on the first and second trailer adapted to detect lateral acceleration and/or wheel speed. In the event that one of the sensors detects lateral acceleration and/or a wheel speed indicative of a loss of stability, the sensor generates a signal for actuating stability control, which signal is passed via the communication interface to the braking ECU on the other trailer, so that the other trailer can actuate stability control.
This system is adapted to measure the actual lateral acceleration or wheel speed of a trailer. If the ECU of one trailer detects a predefined condition indicative of a stability problem, a signal is sent to the ECU of the other trailer, such that the ECU of the other trailer can actuate the brakes of that trailer. This system is thus adapted to control actual, measured conditions when they already have occurred.
WO 2010087022 describes a behaviour controller of a vehicle combination (tractor+trailer/semi trailer) for preventing a jack-knife phenomenon while taking account of a fact that relative rotary action of the trailer and tractor changes according to the vehicle speed or the magnitude of deceleration. The controller comprises a brake force/drive force control section for controlling the brake force/drive force of a tractor or a trailer so as to reduce the difference in yaw rate between the tractor and the trailer. The brake force is controlled when the size of deviation of yaw rate of the tractor and yaw rate of the trailer exceeds a specified threshold. The brake force of each tractor or trailer wheel can be controlled independently.
This system is adapted to measure the actual rotary action between a tractor and a single trailer. This system is thus adapted to control actual, measured conditions when they already have occurred.
In some cases, there may however be an advantage in predicting the movement of the towed vehicles in advance in order to limit the swaying or swinging of the towed vehicles during e.g. a lane change. There is thus still room for improvements.
It is desirable to provide an arrangement for improving stability of a vehicle combination comprising a towing vehicle and at least one towed vehicle. It is also desirable to provide a method for improving stability of a vehicle combination comprising a towing vehicle and at least one towed vehicle.
In an arrangement for improving stability of a vehicle combination comprising a towing vehicle and at least one towed vehicle, where the at least one towed vehicle comprises at least one actively steered axle and/or an individual brake on at least one axle, where the towing vehicle and the at least one towed vehicle each comprises a yaw rate determining means for determining the yaw rate of the vehicle and the at least one towed vehicle, where the arrangement further comprises a vehicle combination model adapted for determining a desired delay value between the yaw rate of the towing vehicle and the yaw rate of the at least one towed vehicle, the problem is solved in that the arrangement is adapted to stabilize the at least one towed vehicle by using the determined yaw rate of the towing vehicle and the desired delay value for the at least one towed vehicle to establish a desired yaw rate for the at least one towed vehicle, and to control the steered axle and/or the individual brake of the at least one towed vehicle such that the determined yaw rate of the at least one towed vehicle corresponds to the desired yaw rate of the at least one towed vehicle.
By this first embodiment of the arrangement, the arrangement will determine the actual yaw rate of the towing vehicle. The arrangement further comprises a vehicle combination model adapted for determining a desired delay value between the yaw rate of the towing vehicle and the yaw rate of each towed vehicle. By applying this delay value to the yaw rate value of each towed vehicle, each towed vehicle will behave similar to the towing vehicle. The stability of the towed vehicles can thus be improved.
The yaw rate of the towing vehicle can be determined either by estimation or by a measurement. The estimation may be done by using an estimation method based on vehicle combination properties, such as wheel speed of the vehicle, vehicle mass, vehicle length, steering angle etc. Yaw rate measurements are done by using a yaw rate sensor.
In an advantageous development of the inventive arrangement, the amplitude of the desired yaw rate of the at least one towed vehicle and the amplitude of the determined yaw rate of the towing vehicle are proportional when carrying out a manouvre. The ratio between the amplitude values is preferably selected such that the ratio is close to 1, and is preferably in the range between 0.9-1.2. In this way, the damping of the yaw rate for the towed vehicles will improve the stability of the vehicle combination.
It is possible to use a deadband during the stabilizing of the vehicle combination when comparing the determined yaw rate with the reference yaw rate. In this way, small unnecessary adjustments of the vehicle combination are avoided when the vehicle combination travels on a straight road or behaves naturally as desired. Even when a vehicle combination travels straight, the towed vehicles may sway slightly sideways. By using a deadband, there will be no stabilizing of the vehicle combination for small sideways movements in this case. This is especially advantageous when the stabilizing actuators are wheel brakes, since the brakes do not have to be actuated constantly.
In a method for stabilizing a vehicle combination comprising a towing vehicle and at least one towed vehicle, the steps of establishing a desired delay value for the yaw rate between the towing vehicle and the at least one towed vehicle in the vehicle combination by using a vehicle combination model, determining the yaw rate of the towing vehicle, establishing a desired yaw rate value for each towed vehicle by using the measured yaw rate and the established delay value, measuring the actual yaw rate of the at least one towed vehicle, comparing the actual yaw rate with the desired yaw rate, and controlling the yaw rate of a towed vehicle to the desired yaw rate value by controlling steering axles and/or individual brakes on the at least one towed vehicle are comprised.
With the inventive method, a vehicle combination comprising at least one towed vehicle can be stabilized. The proportional factor of the yaw rate can be selected but is advantageously close to one.